1. Field of the Invention
The present invention relates to a solid state image pickup device.
2. Description of the Prior Art
In this type of device, conventionally, there are usually used solid state image pickup devices using X-Y addresses, called a MOS type, CCDs of the interline type, CCDs of the frame transfer type, or the like.
The frame transfer type CCDs among them have a very simple structure as compared with that of the devices of the MOS or interline type since it is not necessary to provide a vertical transfer register in an image pickup part. Therefore, it is possible to integrate a large number of horizontal picture elements corresponding to the horizontal direction of the TV screen. FIG. 1 shows such a conventional frame transfer type CCD which comprises an image sensing part 1 to perform the photoelectric conversion, a memory part 2 as a storage section to temporarily store the charges from the image sensing part, a horizontal shift register 3 to transfer the stored charges from the memory part in accordance with the TV synchronization, and an output amplifier 4 to read out the charges as a voltage signal. A color separation filter necessary for production of color signals is adhered or on-chipped on such a CCD. FIG. 2 shows an example of such a filter. There will now be described hereinbelow a conventional frame transfer type CCD using an R-G-B stripe method, which is said to provide excellent color reproductivity.
In the case where a filter according to the stripe method is used and the number of picture elements in the horizontal direction is about 580 elements, the horizontal transfer frequency is equal to 10.7 MHz. When CCDs equal in numer to these elements are used, in many cases, a luminance signal is ordinarily obtained by allowing the output signal of the CCD to pass through a high-band low-frequency filter (about 3 MHz) as it is, while color signals are derived by performing color signal separation with respect to the repetitive frequency 3.58 MHz of the R-G-B signals by means of respective sample and hold circuits.
In this case, there is no problem with respect to the color signals although a frequency band width of 500 kHz is needed for the NTSC system, since the sampling frequency is 3.58 MHz and the components of the signal band up to its Nyquist frequency can be reproduced in the present embodiment. However, for the luminance signal, although no problem will be caused if the color of an object image is nearly achromatic, in case of an object image having high color saturation, the sampling frequency becomes 3.58 MHz (Nyquist frequency: 1.8 MHz), so that a large amount of fold distortion (aliasing noise) could occur and this causes the picture quality to deteriorate remarkably. To solve such a drawback, it is necessary to set the number of horizontal picture elements at about 770 and to drive the CCD at a frequency of 14 MHz. With a CCD which can cope with 14 MHz, even in the case of the special object image mentioned above, the sampling frequency becomes about 4.77 MHz (Nyquist frequency: 2.4 MHz), thereby enabling the fold distortion (aliasing noise) to be greatly reduced; therefore, no problem will be caused in ordinary image receiving apparatuses.
However, in the case of a CCD to be driven at 14 MHz, the problems which will be mentioned below occur with the horizontal shift register, output amplifier, clock IC, and color separation sample and hold circuit.
First, in order to separate the color signals of R, G and B from the output signal of the CCD, the period of the portion corresponding to the valid signal component of the 14-MHz signal must be long to a certain degree. However, in case of the drive pulse having a duty ratio of 50%, the signal component is ##EQU1## but assuming that the leading and trailing times due to the elements of the drive circuit total about 10 ns, the remaining portion would have been actually about 25 ns. Furthermore, if the leading and trailing edges of the clock are extremely steep, the period of the valid signal portion will have been further shortened since the dark current increases due to occurrence of heat in the shift register. In addition, if the frequency characteristic of the output amplifier is also considered, the valid signal period will have been further shortened. As described above, in the CCD to be driven at 14 MHz, the period of the signal component is thus fairly short, and a variation of sampling pulses due to a variation of elements in the clock generating section or due to a temperature variation is also added; therefore there occurs the drawback that the signal separation is difficult.
There is also the drawback that it is at present fairly difficult to integrate a CCD of 770 elements as a horizontal shift register of an image sensor of the size of, e.g. 1/2 inch.
The incident light upon the image pickup device through a filter like that shown in FIG. 2 and through the optical filter is spatially sampled by the above-mentioned color stripe filter and image pickup device. However, in this case, the spatial frequency component of the incident light corresponding to not lower than 1/2 of the spatial sampling frequency to be determined on the basis of the number of picture elements of the image pickup device and of the pitch of the color stripe filter becomes a cause of fold distortion as described previously. This point will be described in detail hereinbelow with reference to FIGS. 3A, 3B and 3C, in which each axis of abscissa denotes a frequency and each axis of ordinate represents a signal level.
Although the incident light samples on the image pickup device is read out as image pickup signals from the image pickup device due to the photoelectric conversion action or the like, when attention is paid to only the R signal (or to only the G or B signal) among these image pickup signals, its repetitive frequency is 1/3 of the readout frequency. Assuming that this repetitive frequency is f.sub.c, the base band component and side band component of the incident light due to the sampling are as shown in FIG. 3A, wherein the hatched section is called a fold distortion component. When this signal is allowed to pass through a low-pass filter having such a characteristic as shown in FIG. 3B, this fold distortion component remains by being mixed with the base band component, so that this distortion component causes the picture quality to deteriorate remarkably on the display. One of the methods of reducing such a fold distortion is set forth in Japanese Kokai 56-120281. For example, as shown in FIG. 3C, when an achromatic color object image is pictured, if the color separation filter is designed in such a manner that the levels of the dot-sequential signals to be output from the image pickup device are 1:1:1, the side band components are set off, thereby enabling the fold distortion to be reduced.
According to this method, it is possible to reduce the fold distortion with respect to at least the achromatic color screen.
It is of course impossible to obtain such an effect with respect to a screen having high color saturation; however, this point may be neglected since human luminous efficiency, or the luminous factor concerning color, on the high-band side is lower.
However, in the above-mentioned case, a variation of color temperature of the photographic light source due to differences in objects and locations of the photographing causes the levels of dot-sequential signals to be unbalanced, and this results in the occurrence of fold distortion. FIG. 4 shows spectral energies at the color temperatures of, e.g. 3200.degree. K. and 6000.degree. K. FIG. 5 shows a drawback in the case where the color separation filter was designed so that the dot-sequential output signal level becomes constant at, e.g. 3200.degree. K. If the filter is set in such a manner that the levels of R, G and B become 1:1:1 at the color temperature of 3200.degree. K., as shown in FIG. 5, the level on the long wavelength side, namely on the R side, will be weak at 6000.degree. K., while the level on the B side will be strong, so that the vector of the side band will have been deflected on the side of cyan (Cy) as shown in FIG. 5, causing fold distortion to occur.
Furthermore, since the image pickup device generally has a high sensitivity for infrared rays and has a difference with luminous efficiency, an infrared-ray cut filter is provided on the light incident optical path of the image pickup device in order to prevent such difference. However, a variations in thickness these infrared-ray cut filters which occur in the manufacturing process cause a variation in spectral sensitivity characteristic of this filter, which may result in a fluctuation in the R signal level.
As a method of eliminating such a drawback, the following method is known.
Namely, a method is known where a mechanical color temperature correction or compensation filter is used. According to this method, for example, color temperature correction filters for use in daylight, fluorescent lamp light and tungsten light are mainly prepared in general and the above-mentioned correction filters are changed over in accordance with the location where the photograph will be taken. A drawback of this method is that several kinds of correction filters are needed and that it is impossible to completely prevent the occurrence of fold distortion since the levels of the dot-sequential signals are roughly matched.
Moreover, in conventional image pickup devices, there is the drawback that noise is so large that the S/N ratio of the picture image is bad. For example, in a television camera using an image pickup tube of the three-electrode type that is disclosed in Japanese 55-51395, a high-band luminance signal is obtained by adding the R, G and B signals. That is, the outut signals R, G and B of the three electrodes becomes R, G and B as shown in FIG. 6 by the scanning of the electron beam, and the three color signals are synthesized to obtain a luminance signal Y. In this case, since the noise caused in the image pickup tube evenly occurs in both the signal component region and the invalid component region, the noise of the synthesized luminance signal is increased by about .sqroot.3 times, so that there is the drawback that the S/N ratio becomes worse by the amount corresponding to that increased noise.
Furthermore, in such an image pickup device, there is a tendency to lower the power voltage of the signal processor for the purposes of miniaturization and low electric power consumption, and power supply of 5 V is at present generally used. In order to generate a video signal with excellent picture quality using such a low power source, it is necessary to properly process the signal level and to equivalently enlarge the dynamic range of the signal processor. In general, as a method of enlarging the dynamic range in the video signal processing, a circuit to nonlinearly suppress the high-luminance signal, called a KNEE circuit, is known. However, if this KNEE circuit is applied as it is to the signal read out from the image pickup device, there are the drawbacks that fold distortion increases in the luminance signal, while an error in white balance occurs in the color signals, causing the picture quality to deteriorate remarkably.
FIGS. 7A and 7B show graphs to explain the above-mentioned drawbacks in conventional technologies.
FIG. 7A will be now described. In many cases, the output levels of each color signal of the image pickup device differ in dependence upon the color temperature of the light source used for photographing as described previously. It is assumed here that an achromatic color object is photographed as an example for purposes of description and that the levels of the R and G signals at this time are equal and that the level of the B signal is slightly lower than the levels of R and G. Namely, it is assumed that each signal level represents the characteristics of R, G and B as indicated by the solid lines in FIG. 7A to an increase in quantity of light. At this time when the saturation signal level is represented by a symbol of V.sub.sat, the color signals R and G will have been saturated in the quantity of light indicated at point a, an the B signal at point b, since the saturation levels of R, G and B are generally identical. When the KNEE characteristic is added to each of the R, G and B signals at a predetermined signal level V.sub.KNEE in order to widen the dynamic range of the circuit in this state, signals R, G and B become the signals R', G' and B' as shown by the broken lines. Next, when the white balance, i.e. the level adjustment of these R', G' and B' signals, is performed, the levels of the R' and G' signals do not coincide with the level of the .alpha.B' signal (where .alpha. is a coefficient for level adjustment) in the range over the V.sub.KNEE level as shown in FIG. 7B. (V.sub.wc denotes the white clipping level and the signal exceeding this level is cut off in the circuit.)
Consequently, in the signal levels in the hatched section of FIG. 7B, the luminance signal level is not matched, causing fold distortion to occur. In addition, there is the drawback that the white balance of the color signals cannot be obtained.